This invention relates to a flue gas desulfurization process that utilizes limestone to regenerate spent alkaline absorption solution containing sodium sulfite and sodium bisulfite.
Flue gas desulfurization processes that employ alkaline absorption solutions containing both sodium sulfite and sodium bisulfite for removing sulfur oxides like SO.sub.2 from flue gases are well known and are normally operated continuously with the spent absorption solution being regenerated with lime or limestone. Such lime/limestone-sodium salt processes are often termed "double" or "dual" alkali processes in contradistinction to single alkali processes that directly treat the flue gas with lime or limestone.
Double alkali processes that utilize limestone as the regenerating agent are described in U.S. Pat. No. 4,410,500 issued to Wang et al., U.S. Pat. No. 4,431,618 issued to Boward et al., (which concerns process control methods); U.S. Pat. No. 3,848,070 issued to Onozuka et al., U.S. Pat. No. 3,944,649 issued to Field et al. and U.S. Pat. No. 3,989,796 issued to Morita et al.
The limestone regeneration is typically carried out as a continuous operation in a series of stirred tank reactors, with the limestone being contacted with spent absorber solution in the first tank. The regeneration tank circuit is usually operated with a relatively dilute concentration of solids, e.g., about 1-3 wt %, that is a mixture of calcium sulfite/sulfate and unreacted limestone.
One drawback to the use of limestone to regenerate sulfite from the bisulfite formed from absorbed sulfur dioxide is its low reactivity. As a consequence, relatively long residence times are needed in the regeneration tank circuit to achieve efficient limestone utilization, i.e., in excess of 90%.
The above-mentioned Wang et al. and Boward et al. patents employ less than a stoichiometric amount of limestone and therefore are vulnerable to factors that aggrevate limestone's low reactivity, such as coarse particle size of the ground limestone. Any appreciable reduction in the limestone's reactivity will require an unacceptably large volume of reactor capacity due to long residence times required in the regeneration procedure to achieve efficient limestone utilization. Various approaches are described in the prior art for coping with limestone's low reactivity.
The above-mentioned Onozuka et al., Field et al. and Morita et al. patents teach that at least a stoichiometric amount of limestone should be employed in the complete neutrilization of the bisulfite during regeneration. Although excess limestone increases the overall regeneration reaction rate, limestone utilization efficiency usually declines as a consequence, and this adversely affects the economics of the overall FGD process.
Modifications to the conventional stirred tank regeneration circuit are described in several patents as a means of improving the limestone regeneration operation.
U.S. Pat. No. 4,388,282 issued to Chou et al. employs hydroclones to separate coarse unreacted limestone particles from smaller calcium sulfite particles in the stirred tank reactor solids; the limestone particles are recycled for further reaction, to improve overall limestone utilization. Since this separation procedure relies on the existence of a size difference between the limestone particles and the calcium sulfite/sulfate regeneration by-product solids, it may not be effective with finely-ground limestone. This procedure is also complicated by the fact that the by-product solids sizing is influenced by the chemistry and mechanical design of the regeneration tank circuit.
Still another approach to this problem is described in U.S. Pat. No. 4,540,556 and No. 4,462,969 both issued to Wilhelm, which rely on the use of two reaction stages that contain a sludge layer or blanket with about 15-40% solids. While this procedure reduces the overall liquor residence time (and regeneration tank size) required, limestone utilization can still suffer because of the difficulty of providing adequate contact of the liquor with the settled solids blanket which contains unreacted limestone.
A simpler approach is described by Dauerman et al. in U.S. Pat. No. 4,261,962, which dispenses with a multiple tank regeneration circuit and teaches the use of a Y-shaped mixing nozzle to facilitate mixing and reaction of the limestone slurry with the absorber solution during regeneration. The patentees recommend use of a stoichiometric excess of limestone to maximize the advantages of their invention, but such increased limestone use inevitably reduces limestone utilization efficiency.
The present invention provides a simple means for achieving excellent limestone utilization, in excess of 90%, in a conventional stirred regeneration tank circuit, without resorting to use of specially-designed equipment.